1. Field of the Invention
The present invention relates to a process for purification of rare gas. More particularly the present invention relates to a process for efficiently removing impurities contained in rare gases, such as helium, neon, argon, krypton and xenon, by the use of a getter metal to thereby purify the rare gas.
2. Description of Related Arts
Rare gases are usually purified by the use of a getter because they have similar chemical properties. Among rare gases, helium and argon are widely used in the semiconductor fabrication industry which has been greatly developing in recent years, and a higher gas purity is required. Moreover, neon, krypton and xenon are essential for production of special lamps, and they are often recycled after being used once, for reuse thereof, because they are very expensive. In this case, it is required that the recycled gas is highly purified by removing the impurities contained therein. Thus, it is desired that nitrogen, hydrocarbon, carbon monoxide, carbon dioxide, oxygen, hydrogen, water, etc., present in the ppm order in the rare gas be removed to the ppb order.
Among rare gases, argon is used in an extremely large amount and, therefore, purification of argon has been mainly under investigation. It is well known that technical information obtained by investigation of the purification of argon is also applicable to purification of other rare gases. There are two types of getters: a vaporization type in which barium, for example, is used, and a non-vaporization type in which titanium, zirconium or the like is used. For the purification of rare gas, the non-vaporization type of getter is usually employed.
Heretofore, as a purification method using the nonvaporization type of getter, there has been employed a method in which nitrogen, hydrocarbon, carbon monoxide, carbon dioxide, oxygen, hydrogen, water, etc., are removed by the use of titanium or a titanium-base alloy. In this method, since it is necessary to maintain the getter at a temperature as high as about 1,000.degree. C., materials for use in, for example, a purification cylinder are substantially limited to quartz, from a viewpoint of thermal resistance. Moreover, when the purification cylinder is used under pressure, there is the danger of it breaking; that is, there is a safety problem. For this reason, the number of purification apparatuses using titanium or a titanium alloy is decreasing, and attempts to develop a purification apparatus using zirconium or its alloy as the getter have been made. Use of zirconium or its alloy as the getter enables lowering the operating temperature of the purification apparatus to 400.degree. to 700.degree. C., and employing stainless steel, etc., for production of the purification apparatus, thereby producing the advantages that the danger of operation under pressure is eliminated and at the same time, an ability to remove hydrogen, which is barely removed at high temperatures, is increased
Japanese Patent Application Laid-Open No. 3008/1987 for example, discloses a Zr-V-Fe three-component alloy as a getter, and a purification apparatus using the threecomponent getter, which permits purification at lowered temperatures. However, since the three-component getter is relatively poor in its ability to remove impurities, specifically nitrogen and hydrocarbon, it has disadvantages in that the apparatus must be increased in size, a large space is needed for setting up the apparatus, and production costs of the apparatus are increased.
Japanese Patent Application Laid-Open No. 118045/1990 discloses a Zr-Al-V three-component alloy getter, and British Patent 1,370,208 discloses a Zr-Ti-Ni three-component alloy getter. These getters have a relatively high ability of removing hydrogen, but have a low ability of removing nitrogen.
Journal of the Less-Common Metals, Vol. 53, pp. 117-131 (1977) describes reversible hydrogen absorption characteristics of three component alloys represented by Zr(Co.sub.x, V.sub.1-x).sub.2 and Zr(Fe.sub.x, V.sub.1-x).sub.2, but there cannot be found any description about the getter action on other gases.
Moreover, as a two component-alloy getter, a Zr-Ti alloy is disclosed in U.S. Pat. No. 2,926,981. In the vicinity of 400.degree. C., the alloy has a higher rate of oxygen absorption than Zr or Ti alone, but it has a disadvantage of being low in absorption rate of other gases.
U.S. Pat. No. 4,071,335 proposes the use of a Zr-Ni alloy. Although the Zr-Ni alloy is excellent in the ability to remove hydrogen and water, its nitrogen removing ability is poor.
U.S. Pat. No. 4,306,887 describes Zr-Fe, which has a markedly poor ability to remove nitrogen.